Acute respiratory distress syndrome (ARDS) affects 100,000-200,000 persons per year in the United States. The large majority of these patients require mechanical ventilation (MV) for survival. However, MV may exacerbate lung injury and delay or prevent recovery from otherwise treatable conditions such as trauma, sepsis, & pneumonia. Several "lung protective" strategies have been examined to limit this ventilator-associated injury (VALI). The use of conventional ventilation with low tidal volumes (Vt) and inspiratory pressure limitation has lead to improved outcomes when compared to more traditional approaches. However, the degree to which Vt can be reduced and still maintain adequate oxygenation and ventilation is limited. An alternative lung protective strategy is high frequency oscillatory ventilation (HFOV), which uses very small Vts at high frequency to achieve effective oxygenation and ventilation. However, little is known about the use of HFOV in adult patients with ARDS. Though a series of small observational studies and one randomized controlled trial suggest that HFOV can be safely administered to patients with ARDS, the physiologic responses to changes in ventilator settings are not well understood. The projects described in this application are designed to establish these relationships so that an efficient and effective protocol may be designed for the comparison of HFOV to the present standard of care among patients ventilated for ARDS.